This invention relates to locking differentials in which the meshing configuration of the teeth of the side gears and pinions of the locked differential are improved.
In our U.S. Pat. No. 4,759,232, Roberts, we have described an improved locking differential capable of remote pneumatic actuation and including a locking ring housed within the differential carrier around one of the bevel gears and connected to a piston actuator located around the other of the bevel gears and connected to the locking ring by a sleeve. This locking system has enjoyed significant commercial success since it provides a locking differential mechanism which can be fitted to an existing differential housing with minimal modification to allow the remote pneumatic operation.
As a result of the extremely confined space within the differential carrier, the actuator and locking ring have always needed to be located on opposite sides of the pinion gear which increases the manufacturing operations needed to produce the locking differential and increases the parts inventory by requiring a connecting sleeve between the locking ring and the actuator. This splitting of the locking mechanism has prevented the use of the same locking system on differentials of smaller dimensions since there is insufficient room between the other bevel gear and the differential carrier to house the actuator mechanism. Also, in some larger differentials the geometry of the differential carrier housing makes it impossible to locate a split locking mechanism of this type.
In our U.S. Pat. No. 5,591,098, we describe improvements in the earlier differential which overcome the above described problem. By providing a modified cover plate forming part of the differential carrier and shaped to house the locking means around the side gear supported by the cover plate, conversion to a locking differential is achieved in a simple and convenient manner which reduces the cost of the conversion and provides a particularly robust conversion for differentials of small dimensions.
While the differential described in the latter patent has also enjoyed commercial success, the meshing positions of the side gears and pinions of the differential are unpredictable when the differential is locked, thereby resulting in the risk of serious tooth damage in the event that locking occurs when the teeth are in an inappropriate meshing relationship. This leads to overdesign of the side gears and pinions with a consequential increase in manufacturing costs.
As depicted in FIGS. 1 and 2 of the accompanying drawings, a dynamic variation exists in the position of the moment of load transfer between any two involute gears in running mesh. The range of values "khgr"1 to "khgr"2 is equivalent to the inclusive radial coordinates that make up the total running surface of the gear. This would suggest that as two gears revolve under uniform load (torque) they undergo a distinct change in the amount and location of stress in the cross section of the load bearing teeth.
When one considers the relative moment of load transfer with respect to the root radius of the tooth profiles (as depicted by values "khgr"1 and "khgr"2 in FIGS. 1 and 2), an even far greater proportional variation of load moment in comparison to that depicted by "khgr"1 and "khgr"2 can be seen. As the root radius of the involute tooth profile is given as the weakest point on the profile (for any given involute pair of like material) then this moment must be considered to be the area of greatest concern to the static loading of involute gears.
The theory behind this concept suggests that since the torque loaded failure of the side gear/pinion gear combinations of the locking differentials described above typically occurs in the root radius of the pinion of a locked assembly, an advantage would be gained in controlling the rotary positions of gears in the locked state such that the best case of load transfer always exists with respect to the root radius of the pinion gear. Given this, and the static load diagrams FIGS. 1 and 2, it is suggested that the rotary relationship depicted by FIG. 2, would be the best position for the gears to be in while under heavy torque load, i.e. in every xe2x80x9clockedxe2x80x9d position in the assembly, the mid-plane of a tooth of the side gear bisects the angle created between the mid-planes of two teeth of the pinion gears
The above theory has been tested using differentials manufactured by the assignee ARB Corporation Limited in accordance with U.S. Pat. No. 5,591,098: the RD15 Airlocker model differentials. The most common type of pinion tooth failure in such differentials when loaded in the xe2x80x9cun-timedxe2x80x9d configuration of FIG. 1, is illustrated in FIG. 3 of the drawings.
It is an object of the present invention to provide a locking differential having an improved side gear/pinion tooth configuration which reduces the likelihood of tooth damage when the differential is subjected to torque in the locked condition.
The invention provides a locking differential including a differential carrier housing a pair of bevel side gears and from two to five pinion gears which mesh with said pair of bevel side gears, locking means positioned within said differential carrier between said differential carrier and one of said bevel side gears, said locking means including locking teeth or splines adapted to engage teeth or splines on said one of said bevel gears to lock the differential, said side gears having teeth in multiples of two to five, depending on the angular positions of the axes of rotation of said pinions with respect to each other, said pinions being mounted on cross shafts, defining said axes of rotation, which are supported by the housing in a fixed position relative to the locking gear such that the teeth of the side gears and the pinion gears are substantially optimally meshed whenever the locking mechanism is in the locked position.
In a preferred embodiment of the invention, each side gear has the same number of teeth as the number of teeth or splines in the locking mechanism, and each pinion has an even number of teeth, and each side gear has a number of teeth which is a multiple of two, three, four or five depending on the angular positions of the axes of rotation of said pinions, the multiple two corresponding to pinions arranged at 180xc2x0 to each other, the multiple three corresponding to pinions arranged at 120xc2x0 to each other, the multiple four corresponding to pinions arranged at 90xc2x0 to each other, the multiple five corresponding to pinions arranged at 72xc2x0 to each other.
It is particularly preferred that the cross shafts are arranged in a fixed position with respect to the position of the locking teeth or splines of the locking mechanism.
The locking mechanism may be in the form of a sliding clutch gear having internal teeth which mesh with the teeth of one of the side gears. In this way, the provision of separate teeth or splines on the side gear is avoided thereby simplifying the side gear design.